1. Field of the Invention
The present invention pertains to a laser detection and ranging (“LADAR”) system and, more particularly, to a LADAR system with lookdown and loitering capabilities.
2. Description of the Related Art
A need of great importance in military and some civilian remote sensing operations is the ability to quickly detect and identify objects, frequently referred to as “targets,” in a “field of regard.” A common problem in military operations, for example, is to detect and identify targets, such as tanks, vehicles, guns, and similar items, which have been camouflaged or which are operating at night or in foggy weather. It is important in many instances to be able to distinguish reliably between enemy and friendly forces. As the pace of battlefield operations increases, so does the need for quick and accurate identification of potential targets as friend or foe and as a target or not.
Remote sensing techniques for identifying targets have existed for many years. For instance, in World War II, the British developed and utilized radio detection and ranging (“RADAR”) systems for identifying the incoming planes of the German Luftwaffe. RADAR uses radio waves to locate objects at great distances even in bad weather or in total darkness. Sound navigation and ranging (“SONAR”) has found similar utility and application in environments where signals propagate through water, as opposed to the atmosphere. While RADAR and SONAR have proven quite effective in many areas, they are inherently limited by a number of factors. For instance, RADAR is limited because of its use of radio frequency signals and the size of the resultant antennas used to transmit and receive such signals. Sonar suffers similar types of limitations. Thus, alternative technologies have been developed and deployed.
One such alternative technology is laser detection and ranging (“LADAR”). Similar to RADAR systems, which transmit and receive radio waves to and reflected from objects, LADAR systems transmit laser beams and receive reflections from targets. Because of the short wavelengths associated with laser beam transmissions, LADAR data exhibits much greater resolution than RADAR data. Typically, a LADAR system creates a three-dimensional (“3-D”) image in which each datum, or “pixel”, comprises an (x,y) coordinate and associated range for the point of reflection.
LADAR systems used for small missile applications are generally mounted at the front of the missile to maximize the collection area for the receiver while maintaining the missile cross section. These LADAR systems use an optical dome which limits the field of regard (“FOR”). Some of these LADARs use a strap-down, staring configuration. In others, gimbals are used to provide stabilization, scan the LADAR transmit beam, and increase the sensor FOR. Traditional gimbal configurations place the gimbal supports along the side of the missile body, using the outer support for elevation and the inner ring for azimuthal motion. Such an arrangement is described in, for example:                (i) U.S. Pat. No. 5,200,606, entitled “Laser Radar Scanning System,” on Apr. 6, 1993, to LTV Missiles and Electronics Group as assignee of the inventors Nicholas J. Krasutsky et al.;        (ii) U.S. Pat. No. 5,224,109, entitled “Laser Radar Transceiver,” on Apr. Jun. 29, 1993, to LTV Missiles and Electronics Group as assignee of the inventors Nicholas J. Krasutsky et al.; and        (iii) U.S. Pat. No. 5,285,461, entitled “Improved Laser Radar Transceiver,” on Feb. 8, 1994, to Loral Vought Systems Corporation as assignee of the inventors Nicholas J. Krasutsky et al.Each of these patents is now commonly assigned herewith.        
Some gimbaling arrangements where the outer gimbal provides the azimuthal motion have been used in belly-mounted configurations. However, this is not compatible with a missile attack scenario and greatly limits the conditions under which the missile can be launched. As LADAR systems and missile weapons systems become more sophisticated, this mission scenario is becoming more common.
Furthermore, providing missiles with designator capabilities has also proved problematic because, unless the missile can loiter and keep the designator beam on target, the time available for designation is very limited. With a front mounted sensor and a gimbal with a limited FOR, the missile can designate as it is flying toward the target. The designator loses sight of the target as the missile passes over it and the missile must turn around to begin designation again. This intermittent designation time is incompatible with the normal operation of a designator which must keep the beam in the target so the attacking missile can track on it.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.